Liquid carbon dioxide recovery from gas mixtures with methane

ABSTRACT

Land-fill and other gases containing principally methane and carbon dioxide have been separated into high BTU fuel gas and discard carbon dioxide containing appreciable methane. Such discard gas can now be simply fractionated into high-purity liquid carbon dioxide with recoveries in excess of 80% while using a single refrigerant at a single low temperature to satisfy all refrigeration requirements of fractionation. This novel fractionation is ideally combined with the process of separating land-fill gas into high BTU fuel gas because then the methane and carbon dioxide are recovered completely as two valuable products, high BTU fuel gas and pure liquid carbon dioxide.

BACKGROUND OF THE INVENTION

In recent years there has been a growing demand for carbon dioxide (CO₂)while economic sources have remained relatively limited. A significantcontributor to the increased consumption of CO₂ is the expanding salesof carbonated beverages including "light beers". While refrigeration isthe leading use of CO₂, the chemical and metal industries are importantbuyers of CO₂. The injection of CO₂ into wells for enhanced oil recoveryis a modern development requiring large quantities of CO₂.

Ammonia plants are estimated to provide by-product CO₂ equivalent toabout 70% of the demand for purified CO₂. Hence, when the sales ofammonia decline, the problem of supplying the users of CO₂ becomesacute.

Accordingly, a principal object of this invention is to open up a newsource of CO₂ heretofore untapped, especially land-fill gas.

Another important object is to provide an economically attractiveprocess for purifying the CO₂ from such new source.

A further object is to achieve very high recoveries of methane andpurified CO₂ from land-fill gas or like gas mixtures of methane and CO₂.

Those and other objects and advantages of the invention will be evidentfrom the description which follows.

SUMMARY OF THE INVENTION

In accordance with this invention, a by-product or discard gas mixtureof at least 90% by volume CO₂ and at least 2% by volume methane iscompressed to a pressure in the range of about 225 to 275 pounds persquare inch absolute (psia), chilled to effect partial liquefaction ofthe gas mixture and fractionated to yield a column bottom liquid productcontaining not less than about 99.5% CO₂, preferably not less than 99.9%CO₂, on a molar basis.

The column top product is a gas mixture of substantially all the methaneentering the column and not more than 20% of the CO₂ entering thecolumn. In other words, the fractionation achieves a CO₂ recovery of atleast 80% as purified liquid.

Traces of other gases which may be present in the CO₂ -methane mixturemay be removed by passing the mixture through adsorbents and/ormolecular sleves which periodically are regenerated or purged as is wellknown in the art.

Preferably, the CO₂ -methane mixture to be treated pursuant to thisinvention for the production of substantially pure liquid CO₂ is theby-product or discard gas of the process for the recovery ofmethane-rich fuel gas from landfill gas as disclosed and claimed in U.S.Pat. No. 4,252,548 to Markbroiter and Weiss. Utilizing the discard gasof that patented process has important advantages including eliminationof any dehydration step before the gas is fractionated, and minimizingthe losses of methane and methanol inherent in the patented process whenthe discard gas thereof is not utilized pursuant to this invention. Inshort, there is synergism in combining the patented process forrecovering methane-rich fuel gas from land-fill gas and the presentinvention of producing substantially pure liquid CO₂ from gas heretoforedischarged into the atmosphere by the patented process.

BRIEF DESCRIPTION OF THE DRAWINGS

The further description of the invention will refer to the appendeddrawings of which:

FIG. 1 is a diagram of a preferred system of the invention forrecovering purified liquid CO₂ from a methane-CO₂ mixture; and

FIG. 2 is a diagram showing the system of the invention integrated withthe system disclosed and claimed in U.S. Pat. No. 4,252,548.

DESCRIPTION OF PREFERRED EMBODIMENTS

Dry by-product gas containing at least 90% by volume CO₂ and at least 2%by volume methane is supplied by line 10 to compressor 11 and passed ata pressure in the range of about 225 to 275 psia by line 12 through aircooler 13. At slightly above ambient temperature the compressed gasflows through line 14, heat exchanger 15, line 16, heat exchanger 17 andline 18 into separator 19 wherein vapor is separated from condensate.For example, if the dry-by-product gas supplied by line 10 came from theprocessing of land-fill gas in accordance with U.S. Pat. No. 4,252,548,it would contain a small fractional percentage, say about 0.2%, ofmethanol. The major portion of the methanol in the gas would in suchcase drop out in separator 19 as condensate and discharge through line20. On the other hand, if the by-product gas was not dry, methanol couldbe injected by line 21 into line 16 and the condensate removed fromseparator 19 by line 20 would be aqueous methanol.

The cold, dry gas passes from separator 19 through line 22 into eitherof two regenerable adsorbers 23A,23B wherein traces of other gases suchas hydrogen sulfide and residual methanol are eliminated from the gasexiting therefrom into line 24. The now substantially pure mixture ofCO₂ and methane flows from line 24, through heat exchanger 25 and line26 into the upper portion of fractionation column 27. Regenerableadsorbers are well known in the art and there is no need to show thevalves which alternately direct the flow of gas from line 22 to one ofadsorbers 23A,23B while the other adsorber undergoes regeneration. As isalso known, the choice of adsorbents and molecular sieves will dependupon the trace contaminants present in the gas. Refrigerant R flowsthrough exchanger 25 to partly liquefy the gas passing through exchanger25.

Vapor leaves fractionator 27 through line 28, passes through refluxcondenser 29 and discharges via line 30 into separator 31. Reflux liquiddrains from separator 31 through line 32 to pump 33 which injects thereflux liquid via line 34 into the upper portion of fractionator 27.Refrigerant R is used in condenser 29 to achieve the desiredcondensation of the vapor passing therethrough. The vapor top product offractionation flows from separator 31 through line 35 to heat exchanger15 wherein it chills the feed gas to be fractionated for the recovery ofpurified liquid CO₂. Thence, the vapor top product containingsubstantially all of the methane in the feed gas leaves through line 36.Column bottom liquid drains from fractionator 27 through line 37 and isdivided into three streams. Two streams are warmed to provide therequired reboiling at the bottom of column 27. The prime or mainreboiler stream is heated by passage through line 38 and exchanger 17before returning via line 39 to column 27 while the auxiliary or trimreboiler stream in line 40 gives up refrigeration in heat exchanger 41to refrigerant R passed therethrough to recover refrigeration beforereturning via line 42 to column 27. The third liquid stream withdrawnfrom line 37 by line 43 passes through heat exchanger 44 wherein it issubcooled by refrigerant R. The subcooled liquid is transferred by line45 as purified liquid CO₂ to storage tank 46. The liquid productreaching tank 46 corresponds to not less than 80% of the CO₂ content inthe feed gas and has a purity of not less than 99.5% CO₂ on a molarbasis.

In FIG. 2, the flow diagram of FIG. 1 is represented by block 50 withline 51 corresponding to line 10, line 52 corresponding to line 36, line53 and tank 54 corresponding to line 45 and tank 46, and line 55corresponding to line 20 of FIG. 1. Block 56 represents the flow diagramof U.S. Pat. No. 4,252,548 with line 57 corresponding to line 10 of thatdiagram, line 58 corresponding to line 22, line 59 corresponding to line54, and line 51 corresponding to the total of lines 42,48 and 53 of thepatented process. Line 60 for supplying fresh make-up methanol to block56 via line 61 corresponds to line 12 of the patented process. Land-fillgas supplied by line 62 is compressed by three-stage compressor 63 andthence introduced by line 57 into block 56 for the recovery of high BTUfuel gas in accordance with the process of U.S. Pat. No. 4,252,548.Inasmuch as gas leaving block 50 through line 52 is under pressure, itis introduced into the last stage of compressor 63 for recycling tomethane purification block 56.

The only addition to the flow diagram of U.S. Pat. No. 4,252,548 is apreliminary partial pressure reduction or expansion of methanolcontaining absorbed gas in line 27 of that diagram to flash off gas(approximately 10% by volume of all the gas scrubbed by methanol)beforethat methanol is passed through reducing valve 28. The flashed gas fromthe preliminary partial expansion is shown in FIG. 2 hereof as gasrecycle via line 64 to the last stage of compressor 63. The addition ofa preliminary partial pressure reduction to the patented processincreases the CO₂ purity of the discard gas passing from block 56through line 51 to block 50. Whereas the total discard gas from lines42, 48 and 53 in the example of U.S. Pat. No. 4,252,548 had a CO₂ purityof 90.6% by volume, the aforesaid preliminary expansion resulting inflashing off some absorbed gas together with recycling the flashed gasto scrubbing with methanol causes the CO₂ purity of the total discardgas to increase to 96.0% by volume. Obviously, the higher the CO₂ purityof the discard gas flowing through line 51 to CO₂ purification andliquefaction block 50 is, the smaller are the equipment and powerconsumption required to produce high purity liquid CO₂.

Refrigeration block 65 highlights another unique benefit of integratingthe process of FIG. 1 with the process of U.S. Pat. No. 4,252,548,namely, a single refrigerant from a common source at a commontemperature is circulated to and from CO₂ purification block 50 vialines 66 and 67 to satisfy all the refrigeration requirements to producepurified liquid CO₂, and the same refrigerant at the same temperature iscirculated to and from methane purification block 56 via lines 68 and69, respectively, to satisfy all the refrigeration requirements toproduce high BTU fuel gas. This is truly an accomplishment of economicsignificance.

When the process of U.S. Pat. No. 4,252,548 is carried out without thebenefit of this invention, residual methanol in the discard gas is lestbut in FIG. 2 it flows through line 51 to CO₂ purification block 50wherein the major portion of the methanol in the discard gas isrecovered as substantially pure methanol and returned via lines 55, 61and 57 to methane purification block 56.

Although the aqueous methanol resulting from the dehydration of theland-fill gas in block 56 may be discarded via line 59, it is advisableto submit the aqueous methanol to rectification, represented in FIG. 2as block 70, so that substantially anhydrous methanol is recovered andreturned by lines 71, 51 and 57 to methane ourification block 56. Theseparated water is discharged from the rectification of block 70 vialine 72.

At a glance, it is evident that substantially all of the land-fill gasentering compressor 63 of FIG. 2 is converted into the high BTU fuel gasof line 58 and the purified liquid CO₂ sent to tank 54. The moistureoriginally present in the land-fill gas becomes the water stream of line72.

As an example of FIG. 1 of the invention, discard gas from the treatmentof land-fill gas in accordance with U.S. Pat. No. 4,252,548 is suppliedby line 10 to compressor 11 which raises the gas pressure to 262 psia.This gas is dry and contains, on a volume basis, 3.77% methane, 96.07%CO₂ and 0.16% methanol. Heat of compression is removed as the gas passesthrough air cooler 13 and enters heat exchanger 15 at a temperature of100° F. Thence, the gas at 81° F. is chilled in exchanger 17 to atemperature of -10° F. with the result that about 60% of the methanol inthe gas condenses and is separated in separtor 19. The chilled gas flowsfrom separator 19 alternately through adsorber 23A or 23B to capture theresidual methanol and trace contaminants. The substantially pure mixtureof methane and CO₂ is then further chilled in exchanger 25 to -30° F. toeffect liquefaction of about 95% of the mixture. The essentially liquidstream discharges from line 26 into the upper portion of column 27maintained at a pressure of 250 psia. Vapor leaving column 27 throughline 28 is chilled led in exchanger 29 to -32° F. to produce refluxliquid which is returned from separator 31 by pump 33 to column 27 vialine 34. The vapor leaving separator 31 through line 35 has acomposition, on a volume basis, of about 25% methane and 75% CO₂. Thisgas at -32° F. is used in exchanger 15 to cool the incoming dischargegas to 81° F. The warmed gas leaving exchanger 15 contains more than99.9% of the methane in the incoming discard gas. Only a trace ofmethane remains in the liquid CO₂ withdrawn from column 27 by line 37.Part of the liquid withdrawn by line 37 passes through exchanger 17 andpart passes through exchanger 41 to effect reboiling in the bottom ofcolumn 27 at a temperature of about - 15° F.

A third part of the liquid in line 37 flows through line 43 andexchanger 44 which subcools the liquid to -20° F. The liquid CO₂discharging into tank 46 has a molar purity of 99.998% and correspondsto 88.2% of the CO₂ present in the incoming discard gas.

The foregoing example makes it clear that a discard gas heretoforevented into the atmosphere is simply convered by the invention intovaluable high-purity liquid CO₂ with an 88.2% recovery of CO₂, thisachievement being all the more noteworthy because a single refrigerantsupplies all the refrigeration required by the process at exchangers 25,29 and 44.

As an example of the integrated system of FIG. 2, undried land-fill gaswith approximately equal parts of methane and CO₂ in line 62 enterscompressor 63 and discharges together with recycle gas from lines 52 and64 at a pressure of 362 psia into line 57. Methanol from line 61 isinjected into the compressed gas to dehydrate the gas as taught in U.S.Pat. No. 4,252,548. The resulting aqueous methanol leaves methanepurification block 56 through line 59 and desirably is rectified inblock 70 to recover substantially anhydrous methanol for return vialines 71 and 61 and reinjection into line 57.

Prior to the first stage of flashing of the partented process, theliquid methanol carrying absorbed CO₂ has its pressure dropped to about122 psia (inlet pressure of the last stage of compressor 63) by passagethrough a reducing valve. The vapor from this preliminary pressurereduction corresponds to about 11% of the gas entering block 56 throughline 57 and is recycled via line 64 to the last stage of compressor 63.The patented methane purification of block 56 delivers to line 58 a highBTU gas containing, on a volume basis, 98% methane and 2% CO₂. The gaspreviously discarded by the process of block 56 is passed by line 51 toCO₂ purification block 50.

Details of the discard gas fed to block 50 and of the CO₂ purificationprocess have already been given in the example of FIG. 1. Over 88% ofthe CO₂ in the discard gas of line 51 is recovered as very pure liquidCO₂ sent to tank 54; however, as will be presently clear, the actualrecovery of CO₂ as purified liquid is about 98% of the CO₂ in theland-fill gas fed by line 62. The remainder of the CO₂ and substantiallyall of the methane in the discard gas is recycled by line 52 to thethird stage of compressor 63. While the recycle gas of line 52 is about13.5% by volume of the discard gas of line 51, it corresponds toslightly less than 8% by volume of the land-fill gas supplied by line62. It is evident in FIG. 2 that all of the methane in the land-fill gasis recovered as high BTU fuel gas except for a trace (not more than 20parts per million) found in the purified liquid CO₂ which corresponds toall of the CO₂ in the land-fill gas except for a small percentage leftin the high BTU fuel gas (98% methane and 2% CO₂ by volume). Aspreviously stated, about 60% of the methanol in the discard gas of line51 is recovered as dry methanol which is recycled via lines 55 and 61for reuse in the dehydration of the land-fill gas flowing through line57. In this example, the single refrigerant, DuPont's Freon-22, flows atthe same temperature of -36° F. from the refrigeration system of block65 to satisfy all the refrigeration requirements of blocks 50 and 56.

Summarizing the synergism achieved by the example of the integratedsystem of FIG. 2, all of the methane in the land-fill gas of line 62except for a trace in the liquid CO₂ product is recovered as high BTUfuel gas containing, on a volume basis, 98% methane and 2% CO₂. Exceptfor the small amount of CO₂ in the high BTU fuel gas of line 58, all ofthe CO₂ in the land-fill gas of line 62 is recovered in tank 54 asliquid CO₂ with a molar purity of 99.998%. In short, the entire contentof methane and CO₂ in the land-fill gas of line 62 is captured in thetwo valuable product streams, namely, high BTU fuel gas of line 58 andvery pure liquid CO₂ of line 53. At the same time, 60% of the methanolthat would have been lost without the process of block 50 is recoveredand recycled for reuse in the process of block 56.

A feature of the invention is that the predetermined low temperature ofthe refrigerant used to satisfy all the refrigeration requirements ofthe processes of blocks 50 and 56 is in the range of about -25° F. to-40° F. Another feature is that the recycle gas of line 52 and therecycle gas of line 64 are usually at sufficient pressure that both needto be compressed in only the last stage of compressor 63. Both featuresenhance the economic attractiveness of the integrated operation of FIG.2.

Many variations and modifications of the invention will be apparent tothose skilled in the art without departing from the spirit or scope ofthe invention. For instance, while land-fill gas is an abundant exampleof gas rich in both methane and CO₂ gases from sewage digestion, coalgasification and synthesis gas generation are likewise rich in methaneand CO₂ and thus can be processed pursuant to FIG. 2 for the recovery ofhigh BTU fuel gas and purified liquid CO₂. As is well known, heatexchanger 17 can be replaced by a coil in the bottom of column 27through which the compressed gas of line 16 would flow and thendischarge via line 18 into separator 19. Similarly, heat exchanger 29can be replaced by a coil in the top of column 27 through whichrefrigerant R would flow to produce reflux liquid within column 27; insuch case, separator 31 and pump 33 would be eliminated. Accordingly,only such limitations should be imposed on the invention as are setforth in the appended claims.

What is claimed is:
 1. The process of recovering liquid CO₂ with a molarpurity of not less than 99.5% from a moisture-free by-product gasmixture of at least 90% by volume CO₂, at least 2% by volume methane anda fractional percentage of methanol, which comprises:compressing saidgas mixture to a pressure in the range of about 225 to 275 psia,chilling the compressed gas mixture containing said methanol by heatexchange with liquid of the reboiler portion of a fractionation zone tocondense a major portion of said methanol, separating the condensedmethanol from the chilled compressed gas mixture, adsorbing tracecontaminants and residual methanol from said chilled compressed gasmixture, further cooling the resulting substantially pure gas mixture ofCO₂ and methane by heat exchange with a refrigerant at a predeterminedlow temperature to effect partial liquefaction of said pure gas mixture,discharging the partially liquefied gas mixture into the reflux portionof said fractionation zone, cooling vapor of the top of saidfractionation zone by heat exchange with said refrigerant at saidpredetermined low temperature to provide reflux liquid for said refluxportion, withdrawing from the top of said fractionation zone a vaporstream containing substantially all of said methane in said by-productgas mixture and not more than 20% of the CO₂ in said by-product gasmixture, and withdrawing liquid CO₂ with a molar purity of not less than99.5% from the bottom of said fractionation zone.
 2. The process ofclaim 1 wherein the liquid CO₂ withdrawn from the bottom of thefractionation zone is subcooled by heat exchange with the samerefrigerant at the same predetermined low temperature used to providereflux liquid.
 3. The process of claim 1 wherein the predetermined lowtemperature of the refrigerant is in the range of about -25° F. to -40°F.
 4. The process of recovering liquid CO₂ with a molar purity of notless than 99.5% from a moisture-containing by-product gas mixture of atleast 90% by volume CO₂ and at least 2% by volume methane, whichcomprises:compressing said gas mixture to a pressure in the range ofabout 225 to 275 psia, injecting methanol into said gas mixture,chilling the compressed gas mixture containing said methanol by heatexchange with liquid of the reboiler portion of a fractionation zone tocondense substantially all the moisture as an aqueous methanolcondensate, separating said condensate from the chilled compressed gasmixture, adsorbing trace contaminants as well as residual methanol andmoisture from said chilled compressed gas mixture, further cooling theresulting substantially pure gas mixture of CO₂ and methane by heatexchange with a refrigerant at a predetermined low temperature to effectpartial liquefaction of said pure gas mixture, discharging the partiallyliquefied gas mixture into the reflux portion of said fractionationzone, cooling vapor of the top of said fractionation zone by heatexchange with said refrigerant at said predetermined low temperature toprovide reflux liquid for said reflux portion, withdrawing from the topof said fractionation zone a vapor stream containing substantially allof said methane in said by-product gas mixture and not more than 20% ofthe CO₂ in said by-product gas mixture, and withdrawing liquid CO₂ witha molar purity of not less than 99.5% from the bottom of saidfractionation zone.
 5. The process of claim 4 wherein the liquid CO₂withdrawn from the bottom of the fractionation zone is subcooled by heatexchange with the same refrigerant at the same predetermined lowtemperature used to provide reflux liquid.
 6. The process of claim 4wherein the predetermined low temperature of the refrigerant is in therange of about -25° F. to -40° F.
 7. In the process for removing CO₂from a feed gas containing at least 50% by volume methane and at least5% by volume CO₂ comprising: compressing said gas; injecting methanolinto said gas to dehydrate said gas by chilling said gas to condense allthe moisture therein as aqueous methanol and by separating said aqueousmethanol from the chilled dehydrated gas; scrubbing said chilleddehydrated gas at a pressure of about 200 to 600 psia with cold recycledmethanol supplied at a temperature of about -40° F. to -70° F. andcontaining not more than about 2% on a molar basis of absorbed CO₂ ;recovering the scrubbed gas containing not more than about 2.5% byvolume CO₂ as high BTU fuel gas product; chilling the methanol withabsorbed CO₂ withdrawn from said scrubbing by heat exchange with arefrigerant at a predetermined low temperature; and passing the chilledmethanol through a pressure-reduction flashing separation of CO₂ -richgas from said methanol and at least two heat flashing separations of CO₂-rich gas from said methanol which is then recycled to said scrubbing;the improvement which comprises:compressing all of the flashed CO₂ -richgas containing at least 90% by volume CO₂, at least 2% by volume methaneand a fractional percentage of methanol to a pressure in the range ofabout 225 to 275 pisa, chilling the compressed CO₂ -rich gas containingsaid methanol by heat exchange with liquid of the reboiler portion of afractiontion zone to condense a major portion of said methanol,separating the condensed methanol from the chilled compressed CO₂ -richgas, adsorbing trace contaminants and residual methanol from saidchilled compressed CO₂ -rich gas, further cooling the resultingsubstantially pure gas mixture of CO₂ and methane by heat exchange withsaid refrigerant at said predetermined low temperature to effect partialliquefaction of said pure gas mixture, discharging the partiallyliquefied gas mixture into the reflux portion of said fractionationzone, cooling vapor of the top of said fractionation zone by heatexchange with said refrigerant at said predetermined low temperature toprovide reflux liquid for said reflux portion, withdrawing from the topof said fractionation zone a vapor stream containing substantially allof said methane in said flashed CO₂ -rich gas and not more than 20% ofthe CO₂ in said flashed CO₂ -rich gas, and recycling said vapor streamfor compression with said feed gas; and withdrawing liquid CO₂ with amolar purity of not less than 99.5% from the bottom of saidfractionation zone.
 8. The improvement of claim 7 wherein the condensedmethanol separated from the chilled compressed CO₂ -rich gas is recycledfor injection into the compressed feed gas for the dehydration thereof.9. The improvement of claim 7 wherein the liquid CO₂ withdrawn from thebottom of the fractionation zone is subcooled by heat exchange with thesame refrigerant at the same predetermined low temperature used toprovide reflux liquid.
 10. The improvement of claim 7 wherein thepredetermined low temperature of the refrigerant is in the range ofabout -25° F. to -40° F.
 11. The improvement of claim 7 wherein prior topassing the chilled methanol with absorbed CO₂ through thepressure-reduction flashing separation of CO₂ -rich gas from saidchilled methanol, said chilled methanol with absorbed CO₂ is passedthrough a preliminary partial pressure-reduction flashing separation ofCO₂ -rich gas which is recycled for scrubbing with cold methanol. 12.The improvement of claim 11 wherein the CO₂ -rich gas from thepreliminary partial pressure-reduction flashing separation amounts toabout 10% by volume of all the gas scrubbed with cold methanol.
 13. Theimprovement of claim 11 wherein the liquid CO₂ withdrawn from the bottomof the fractionation zone is subcooled by heat exchange with the samerefrigerant at the same predetermined low temperature used to providereflux liquid.
 14. The improvement of claim 13 wherein the predeterminedlow temperature of the refrigerant is in the range of about -25° F. to-40° F.